The invention relates to digital voice enhancement, DVE, communication systems, and more particularly to enhanced selection techniques between microphones.
The invention may be used in duplex systems, for example as shown in U.S. Pat. No. 5,033,082, and U.S. application Ser. No. 08/927,874, filed Sep. 11, 1997, simplex systems, for example as shown in U.S. application Ser. No. 09/050,511, filed Mar. 30, 1998, all incorporated herein by reference, and in other DVE communication systems.
The invention of the ""874 application relates to acoustic echo cancellation systems, including active acoustic attenuation systems and communication systems. The invention of the ""874 application arose during continuing development efforts relating to the subject matter of U.S. Pat. No. 5,033,082, incorporated herein by reference.
In one aspect of the invention of the ""874 application, a fully coupled active echo cancellation matrix is provided, cancelling echo due to acoustic transmission between zones, in addition to cancellation of echoes due to electrical transmission between zones as in incorporated U.S. Pat. No. 5,033,082. In the latter patent, a communication system is provided including a first acoustic zone, a second acoustic zone, a first microphone at the first zone, a first loudspeaker at the first zone, a second microphone at the second zone and having an output supplied to the first loudspeaker such that a first person at the first zone can hear the speech of a second person at the second zone as transmitted by the second microphone and the first loudspeaker, a second loudspeaker at the second zone and having an input supplied from the first microphone such that the second person at the second zone can hear the speech of the first person at the first zone as transmitted by the first microphone and the second loudspeaker, a first model cancelling the speech of the second person in the output of the first microphone otherwise present due to electrical transmission from the second microphone to the first loudspeaker and broadcast by the first loudspeaker to the first microphone, the cancellation of the speech of the second person in the output of the first microphone preventing rebroadcast thereof by the second loudspeaker, and a second model cancelling the speech of the first person in the output of the second microphone otherwise present due to electrical transmission from the first microphone to the second loudspeaker and broadcast by the second loudspeaker to the second microphone, the cancellation of the speech of the first person in the output of the second microphone preventing rebroadcast thereof by the first loudspeaker. In the invention of the ""874 application, there is provided a third model cancelling the speech of the first person in the output of the first microphone otherwise present due to acoustic transmission from the second loudspeaker in the second zone to the first microphone in the first zone, and a fourth model cancelling the speech of the second person in the output of the second microphone otherwise due to acoustic transmission from the first loudspeaker in the first zone to the second microphone in the second zone. The invention of the ""874 application has desirable application in those implementations where there is acoustic coupling between the first and second zones, for example in a vehicle such as a minivan, where the first zone is the front seat and the second zone is a rear seat, and it is desired to provide an intercom communication system, and cancel echoes not only due to local acoustic transmission in a zone but also global acoustic transmission between zones, including in combination with active acoustic attenuation.
In another aspect of the invention of the ""874 application, there is provided a switch having open and closed states, and conducting the output of a microphone therethrough in the closed state, a voice activity detector having an input from the output of the microphone at a node between the microphone and the switch, an occupant sensor sensing the presence of a person at the acoustic zone, and a logical AND function having a first input from the voice activity detector, a second input from the occupant sensor, and an output to the switch to actuate the latter between open and closed states. This feature is desirable in automotive applications when there are no additional passengers for a driver to communicate with.
In another aspect of the invention of the ""874 application, an input to a model is supplied through a variable training signal circuit providing increasing training signal levels with increasing speech signal levels or increased interior ambient noise levels associated with higher vehicle speeds. This is desirable for on-line training noise to be imperceptible by the occupant yet have a sufficient signal to noise ratio for accurate model convergence.
In another aspect of the invention of the ""874 application, a noise responsive high pass filter is provided between a microphone and a remote yet acoustically coupled loudspeaker, and having a filter cutoff effective at elevated noise levels and reducing bandwidth and making more gain available, to improve intelligibility of speech of a person in the zone of the microphone transmitted to the remote loudspeaker. In vehicle applications, the high pass filter is vehicle speed sensitive, such that at higher vehicle speeds and resulting higher noise levels, lower frequency speech content is blocked and higher frequency speech content is passed, the lower frequency speech content being otherwise masked at higher speeds by broadband vehicle and wind noise, so that the reduced bandwidth and the absence of the lower frequency speech content does not sacrifice the perceived quality of speech, and such that at lower vehicle speeds and resulting lower noise levels, the cutoff frequency of the filter is lowered such that lower frequency speech content is passed, in addition to higher frequency speech content, to provide enriched low frequency performance, and overcome objections to a tinny sounding system.
In another aspect of the invention of the ""874 application, there is provided a feedback detector having an input from a microphone, and an output controlling an adjustable notch filter filtering the output of the microphone supplied to a remote yet acoustically coupled loudspeaker. This overcomes prior objections in closed loop communication systems which can become unstable whenever the total loop gain exceeds unity. Careful setting of system gain and acoustic echo cancellation may be used to ensure system stability. For various reasons, such as high gain requirements, acoustic feedback may occur, which is often at the system resonance or where the free response is relatively undamped. These resonances usually have a very high Q factor and can be represented by a narrow band in the frequency domain. Thus, the total system gain ceiling is determined by a small portion of the communication system bandwidth, in essence limiting performance across all frequencies in the band for one or more narrow regions. The present invention overcomes this objection.
In another aspect of the invention of the ""874 application, an acoustic feedback tonal canceler is provided, removing tonal noise from the output of the microphone to prevent broadcast thereof by a remote but acoustically coupled loudspeaker.
The invention of the ""511 application arose during development efforts directed toward reducing complexities of full duplex voice communication systems, i.e. bidirectional voice transmission where talkers exchange information simultaneously. In a full duplex system, acoustic echo cancellation is needed to overcome feedback generated by closed loop communication channel instabilities. Use of a simplex scheme that alternately selects one or another microphone or channel as active is another way to effectively control feedback into a near end microphone from a near end loudspeaker. In a simplex system, voice transmission is unidirectional, i.e. either one way or the other way at any given time, but not in both directions at the same time.
A simplex digital voice enhancement communication system does not rely on acoustic echo cancellation to ensure stable communication loop gains for closely coupled microphones and loudspeakers. However, there is a potential for feedback into a near end microphone from a far end loudspeaker. This situation exists because it would be self-defeating to have the active microphone switched off. The invention of the ""511 application addresses and solves this problem in a particularly simple and effective manner with a combination of readily available known components.
The present invention relates to enhanced selection techniques in a digital voice enhancement communication system for selecting which of a plurality of microphones to connect to a loudspeaker. The switch in the DVE system must decide which microphone from an array of microphones to select as the active one. In the past, this decision was done by comparing the average magnitude of all microphone signals in which speech was detected (voice plus noise signals). The accuracy of this method was dependent on the sensitivity of each microphone and the background (noise) signal levels at each microphone. For example, a first talker might have a more sensitive microphone than a second talker and would therefore have a higher chance at being selected as the active talker. As another example, a third talker might be in a noiser location and therefore have a higher chance at being selected. The noted prior art method was not immune to different microphone sensitivities and different background noise levels. The present invention addresses and solves this problem.